As is known, the digital computer has become the most economic and efficient piece of equipment in the processing and instrumentation fields. However, its adaptation to analog input signals has presented many problems. Rejection of common mode and normal mode noise brought about by industrial environments and system imbalance is of major importance to the computer interface. Additionally, converting the analog transducer signal to a digital code within system accuracy is of equal importance.
High voltage common mode noise can destroy or damage much of the costly precision instrumentation at the computer interface if not rejected. Furthermore, this same noise will create normal mode noise due to system inaccuracies (i.e., imbalance of transducer impedance, improper cabling and terminations and the like). The normal mode noise can yield misleading output information. In control systems where a monotonically increasing or decreasing signal is demanded, an erroneous signal due to persistent noise can prove disastrous.
Prior art protection systems for computer interfaces against common mode noise comes from a fully-guarded floating metal enclosure which completely surrounds the precision instrumentation. The transducer analog signal is presented to the interface through a twisted-shielded cable and is usually connected to the bus by a three-pole electromechanical relay. The shield is connected to the metal enclosure. Due to capacitive coupling to system ground and leakage caused by slight conductance of insulation, currents will be allowed to flow in the shield. This will induce normal mode noise into the signal input. To eliminate this noise, the input signal is usually amplified and converted to a rate signal which is coupled to another system which integrates the signal over one period of line frequency. Phase locking onto line frequency is the method used for determining the precise period of integration.
Such prior art systems employ complex electronic circuitry to meet their high standards. Multiplexing of the analog inputs has become mandatory to reduce costs per analog input signal. It can be readily seen that as the number of analog signals with which the computer must interface is reduced to a small number, the economics of the system becomes the dominant factor.